In littoral sediments, microphytobenthic (MPB) nitrogen assimilation often exceeds nitrogen removal by denitrification, partly because MPB activity suppresses denitrification. Little is known about the balance between these two processes at sublittoral depths. Benthic pigment composition, light and dark oxygen, and nutrient fluxes (NO 3 Ϫ , NH 4 ϩ , dissolved organic nitrogen (DON), PO , Si(OH) 4 ), as well as denitrification were measured between 1 and 3Ϫ 4 15 m in depth in Gullmar Fjord (Skagerrak) in spring and autumn. The hypothesis was that the assimilation/ denitrification ratio would decrease with depth, along with decreasing MPB activity caused by light limitation. MPB photosynthesis occurred along the entire depth gradient, although sediments were net autotrophic only above 5 m. Inorganic nitrogen (DIN) (and silica) flux changed along the depth gradient, the general pattern being sediment uptake at Յ5 m and efflux at Ն10 m depth. DON flux (ϳ50% of total dissolved nitrogen flux) showed a less clear pattern. Two trends regarding DIN fluxes and denitrification-significant light effects and negative correlations with gross primary productivity-showed that MPB activity influenced nitrogen (N) turnover. Although denitrification increased with depth, rates remained low (Ͻ0.4 mmol N m Ϫ2 d Ϫ1 ), and MPB assimilation (0.2-3.6 mmol N m Ϫ2 d Ϫ1 ) exceeded or equaled denitrification. MPB incorporated ϳ35% of the remineralized N along the depth gradient, whereas denitrification removed ϳ20%. Thus, the influence of MPB on benthic nitrogen turnover, denitrification included, extends to sublittoral depths. Further, denitrification does not necessarily remove more N in the deeper, heterotrophic part of the photic zone, compared to the littoral, autotrophic zone.

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Although it is established that labile fractions of dissolved organic nitrogen (DON), such as dissolved free amino acids (DFAA), can be utilized by microalgae, few studies have considered their quantitative importance for microphytobenthos (MPB). Mixed MPB communities, from 1 and 20 m water depth and treated in 2 ways (semi-natural and isolated), and axenic diatom cultures (Cylindrotheca fusiformis) were examined for the uptake of DFAA. Uptake was measured using naturally low concentrations (<1 µmol l -1 ) of a mixture of 3 H-labeled DFAA ( 3 H-AA) under different light conditions and varying levels of dissolved inorganic nitrogen (DIN). Prokaryotic and eukaryotic uptake were differentiated using chloramphenicol. Biomass (chl a) and production ( 14 C uptake) were measured as well. 3 H-AA was taken up during the incubations in all experiments: 20 to 40% in the culture experiments, and 2 to 18% in the mixed MPB experiments. Of the total adsorption-corrected uptake, prokaryotic uptake accounted for 45 to 85% (0.3 to 2.2 µmol N m -2 h -1 ), while eukaryotic uptake accounted for 15 to 55% (0.3 to 0.7 µmol N m -2 h -1 ). Short-term (hours) light changes had no effect on 3 H-AA uptake, and ammonium availability lowered 3 H-AA uptake only marginally. Shadeadapted MPB communities (20 m) took up 3 H-AA more efficiently than light-adapted communities (1 m). The estimated contribution from DFAA to the microalgal N-demand was 6 to 12 and 55 to 100% (1 and 20 m sediment, respectively). Our findings suggest that DON, specifically DFAA, may be an important source of N for MPB communities, particularly where DIN and light are limiting. Moreover, tentative estimations suggest that DFAA can provide MPB communities with a considerable portion of their total N-demand (10 to 100%).

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